This invention relates to improvement in polyester yarn, and is more particularly concerned with providing new yarns of polyester fibers that may be continuous or cut and that have an improved cross-section in that the periphery of the new cross-section is a simple oval contour that provides advantages in improved dye yield, and also in aesthetics, and in open-end spinning of such cut fibers, and that may be in the form of spun yarns prepared from spinning such new fibers, and in downstream products of such fibers and yarns, and in processes for obtaining such fibers, yarns and downstream products.
Most synthetic yarns, including yarns of polyester fibers, can be classified into two groups, namely (1) continuous filament yarns and (2) so-called xe2x80x9cspun yarnsxe2x80x9d of fibers that are discontinuous, which latter are often referred to as staple fibers or cut fibers.
In relation to the latter group, polyester staple fibers are first formed by extrusion into continuous polyester filaments, which are processed in the form of a tow of continuous polyester filaments, before the filamentary tow is converted into staple, which is then spun into spun textile yarn, often from blends of polyester fiber with other fibers, mostly cotton fibers or other natural and/or synthetic fibers.
Virtually all polyester staple fibers used to make commercial yarns for the apparel market (except for those in some selected specialty applications) have been of round cross-section for practical and economic reasons. The cross-sectional shape is established by the fiber producer primarily during melt-spinning and is then essentially fixed during drawing and annealing steps used to strengthen the fiber and to stabilize the fine structure of the polyester. Once established by the fiber producer, the cross-section of staple fiber generally remains essentially unchanged during subsequent mill processing steps used to form the yarns, fabrics and garments. Increasing the complexity of the cross-sectional shape (i.e., making and using any cross-section other than round) has generally increased processing difficulty and costs for fiber producers and especially for fiber processors.
Most commercially available continuous filament yarns have also been of round cross-section, although there have been various suggestions, especially in the patent literature, of spinning specific non-round filaments.
Both terms xe2x80x9cfiberxe2x80x9d and xe2x80x9cfilamentxe2x80x9d are used generically herein to include both continuous filaments and staple fiber (cut fiber) unless continuous filaments or staple fiber (cut fiber) are specifically mentioned.
Fiber producers prefer to manufacture round fibers over non-round fibers because melt-spinning (extruding) round filaments is most efficient and economical. Round orifices can be easily and economically fabricated. Further, melt-spinning processes used for round filaments are less demanding than for non-round filaments in that filament formation requires less strict control of polymer viscosity and air quenching to achieve acceptable quality. Immediately after extrusion, the melt tends to swell and form a bulge under the capillary orifice. Additionally, the uniform and symmetrical surface of the round shape minimizes directional influences during the filament-forming operation and maximizes the opportunity for increasing uniformity of fiber tensile, crimp and lubrication properties, uniformity generally being highly desirable.
Likewise, textile processors have preferred to process round fibers over non-round fibers in their normal processing operations. It is especially true that round fibers have been easier and more cost-efficient to transform into spun yarns and fabric; this has been the case particularly in the textile operations of carding, drafting and spinning used to transform the raw cut polyester staple fiber into spun textile yarn. No doubt this has resulted partly from the better property uniformity as discussed above and partly from the uniform friction and processing characteristics of the symmetrical round surface.
Round fibers have also been highly desirable for their economic dyeability and coloring characteristics. Of all potential cross-sections, round fibers possess minimum surface area to color and, therefore, require less dyestuff for coloration, in contrast to any non-round cross section which must necessarily have increased surface area, so would be expected to dye with lower yield and, therefore, generally require a higher level of costly dyestuffs to achieve the same coloration as a round cross-section.
Textile designers have always been searching for ways to vary aesthetics of fabrics. Different cross-sectional shapes of fibers could provide different aesthetics in fabrics and garments. The economic penalty required to dye non-round synthetic fibers, however, has been a serious disadvantage heretofore.
As indicated, both fiber producers and textile mill operators have been driven by economic considerations, so polyester fibers with non-round cross-sections have found little to no use in high volume commodity blends applications, especially in polyester/cotton for the commodity apparel market. The few examples of non-round fibers in the apparel market have been limited to specialty fibers that have provided marketable visual and/or performance fabric and garment attributes that have commanded point of sales premiums to off-set the necessary added producer and textile mill costs.
This invention, in contrast, provides a commodity polyester fiber of non-round cross-section that provides, surprisingly, yarns and fabrics that have shown dye yields equivalent or near equivalent to round fibers, as well as other advantages, including improved open-end spinning performance over round fibers, as will be explained hereinafter.
According to one aspect, the present invention provides improved yarn comprising polyester fiber of simple oval cross-section of aspect ratio about 1.85:1 to about 3.5:1. xe2x80x9cSimplexe2x80x9d oval cross-section is discussed and distinguished from a more complex oval cross-sectional shape hereinafter. Preferably, the aspect ratio is at least about 2.0:1, especially about 2.1-2.5:1.
We have found that yarns and fabrics of such polyester fibers can be dyed with little or no loss in coloration (i.e., dye yield) using the same weight percent of dyestuff as the commodity round fibers. In contrast, fibers with other oval cross-sections have dyed significantly lighter than round fibers when the same amount of dyestuff has been provided for such fibers, as will be discussed hereinafter.
We have also found that such fibers in the form of staple (cut fiber) have provided efficiency gains in open-end spinning as compared with conventional fibers of round cross-section, by reducing spinning interruptions at fixed processing speeds, or by allowing processing speeds to be increased without exceeding normal mill accepted interruption level with a consequent gain in mill productivity.
According to further aspects of the invention, therefore, there are provided a process of open-end spinning polyester staple fiber alone, or mixed with cotton, said polyester fiber having a simple oval cross-section of aspect ratio about 1.85:1, and preferably at least about 2.0:1, to about 3.5:1, and especially about 2.1-2.5:1, and open-end spun yarns of polyester staple fiber having a simple oval cross-section of aspect ratio from about 1.85:1, and preferably at least about 2.0:1, to about 3.5:1, and especially about 2.1:2.5:1, either alone or mixed with cotton.
Also provided according to the invention are fabrics and garments of such new yarns.